This invention relates to a process for reducing the concentration of nitrogen oxides contained in a gaseous mixture. In particular, this invention relates to a process wherein the concentration of nitrogen oxides is reduced by catalytic reduction.
Nitrogen oxides are, of course, generally present in significant quantities in gaseous mixtures such as flue gases. Different methods have been used in the treatment of these gas mixtures. One type of treatment involves the catalytic reduction of the nitrogen oxides. As typical processes for removing nitrogen oxides from flue gas by catalytic reduction, there can be mentioned two processes: (1) a nonselective reduction process wherein carbon monoxide, hydrogen or a lower hydrocarbon is used as the reducing agent and (2) a selective reduction process wherein ammonia is used as the reducing agent. In the latter process (selective reduction process with ammonia), a high degree of removal of nitrogen oxides can be obtained with a small amount of reducing agent. Therefore, this process has become of major interest and several variations have been proposed.
The processes for catalytic reduction of nitrogen oxides with ammonia as the reducing agent which have been proposed so far can be divided roughly into two groups; (1) processes using a catalyst wherein the active ingredient is a noble metal such as platinum, or palladium and (2) processes using a catalyst wherein the active ingredient is a compound of a base metal, particularly a non-noble transition metal, such as copper, iron, vanadium, chromium and molybdenum. The active ingredients of these catalysts are carried generally on alumina. Noble metal catalysts are defective in that they are seriously poisoned by sulfur oxides contained in the exhaust gas. On the other hand, the base metal catalysts, while they have good resistance to poisoning by the sulfur oxides, are less active in the catalytic reduction of nitrogen oxides. Therefore, elevation of reaction temperature and reduction of space velocity are required. Exhaust gas to be treated is large in amount and temperature of the exhaust gas is low in general. Therefore, development of a highly active catalyst which can be used under reaction conditions of a low temperature and a high space velocity is desired.